1. The Field of the Invention
This invention relates to the optical transformation of the shape of a nominally circular object to an object with non-unity aspect ratio without substantial loss of brightness. When the aspect ratio of the output is chosen to be high, the resultant shape is particularly appropriate as an input to a diffraction-grating based spectrometer.
2. Background and Relevant Art
When an object is illuminated with a beam of optical radiation for the purpose of gathering light from the object, the optimum shape of the illumination beam is often nearly circular. If for example, the object has volumetric scattering properties, and it is desired to observe the back-scattered radiation, for an imaging system with limited field of view, the optimum geometry for collecting the largest proportion of back-scattered light is to image an area centered on a circular illumination beam.
If it is desirable to subject light which has either been scattered by or transmitted by an object to spectroscopic analysis, and the spectrometer is based on diffraction, it is advantageous to present a nominally rectangular input to the spectrometer of high aspect ratio, where the short axis of the rectangle is in the direction of the dispersion of the spectrometer. Such an arrangement optimizes the resolution of the spectrometer. Hence, if it is advantageous to both collect light from an area having near unity aspect ratio and present a high aspect ratio input of nominally rectangular shape to the spectrometer, it is desirable to find an arrangement which optically converts a near unity aspect ratio spot to a high aspect ratio output. There can, of course, be other circumstances besides spectroscopy where such an optical conversion can be advantageous, and no limitation to spectroscopy for the usefulness of such a converter is implied.
The conversion of near-unity aspect ratio spots to high aspect ratio outputs is sometimes accomplished with a fiber bundle, where the fibers are arrayed in a close packed geometry at the end where light is to be collected and re-arranged in a linear configuration at the opposite end. Such an arrangement will reduce the collection efficiency by approximately the ratio of the area of the fiber cores to the area of the collection bundle. In addition, it is difficult to re-arrange the close packed configuration at one end to the nominally linear configuration at the other end in a short length, hence it is difficult to make the fiber bundle devices compact.
Anamorphic prism pairs have been used for such transformations wherein a nominally collimated beam passes through a first prism and is deflected in angle, thereafter, passing through a second prism which is disposed at an appropriate angle with respect to the first. It is difficult to obtain changes in aspect ratio of more than a factor of five with this arrangement and it is also very difficult to reduce reflection losses at all surfaces and in all polarizations because the beams can be incident at high angles. Moreover, this approach requires a collimated beam which makes the embodiment less compact.
Another optical scheme for performing aspect ratio transforms is an anamorphic lens system as is described in “Anamorphic Condensing Optics for a Slitless Spectrograph”, J. R. Baskett, and I. D. Liu, Applied Optics, 9, p. 49-52 (1970). The lens system consists of a spherical lens, followed by a cylindrical lens, followed by a second spherical lens, concluding with a second cylindrical lens. The system is complex, and is not suitable for high numerical aperture applications, and also consumes considerable space.
In yet another approach a non-imaging elliptical concentrator is proposed in, “Elliptical Concentrators”, A. G. Garcia-Botella et al., Applied Optics, 45, p. 7622-7627 (2006). The difficulty with such concentrators is that the angular distribution of the input radiation is not preserved, with angular divergences in the two planes scaling inversely with geometrical scaling in the two planes. In many applications the input radiation diverges symmetrically and it is desired that the output radiation nominally preserve this property.
Finally, it is possible to design a diffractive element that provides both wavelength dispersion and transformation of aspect ratio. Such a design is described in “Lensless anamorphic Fourier transform hologram recorded with prism systems”, A. Viswanath and K. Srinivasan, Applied Optics, 36, p. 5748-5755 (1997). It is difficult to simultaneously obtain high diffraction efficiency, wide field of view, and high numerical aperture with this design.